How to Handle File Downloads of Unknown Types in ASP.NET MVC using FileResult?

Using FileResult in ASP.NET MVC to download unknown type files

When enabling file downloads in ASP.NET MVC, it is generally recommended to use FileResult as the preferred method. However, most examples focus on downloading image files with a specific content type (such as "image/jpeg").

A problem arises if the file type is unknown and you want to allow users to download various file formats: the downloaded file name may be concatenated with underscores from the file path, which may not be ideal. Also, some people prefer to return a FileResult instead of using a custom class such as "BinaryContentResult".

Recommended method

In order to resolve these issues and implement the correct download mechanism, please follow these steps:

1. Use the generic octet stream MIME type:

   For files of unknown type, specify the generic "Application/Octet-Stream" MIME type to indicate that the file can be in any format.

2. Define download operation:

   In your MVC controller, create an action method to handle the file download, for example:

   public FileResult Download(string filePath, string fileName)
   {
       // 从指定位置或流获取文件字节
       byte[] fileBytes = GetFileBytes(filePath);
   
       // 使用指定的字节、MIME类型和文件名返回FileResult
       return File(fileBytes, System.Net.Mime.MediaTypeNames.Application.Octet, fileName);
   }

3. Retrieve file bytes:

   Implement a method that retrieves file bytes based on a provided file path or other means, for example:

   private byte[] GetFileBytes(string filePath)
   {
       return System.IO.File.ReadAllBytes(filePath);
   }

Sample code:

Here is a sample code snippet demonstrating the approach:

public ActionResult Download()
{
    byte[] fileBytes = System.IO.File.ReadAllBytes(@"c:\folder\myfile.ext");
    string fileName = "myfile.ext";
    return File(fileBytes, System.Net.Mime.MediaTypeNames.Application.Octet, fileName);
}

With this method you can download any type of file while keeping the desired filename and using the appropriate MIME type in your ASP.NET MVC application.

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